1. Field of the Invention
This invention relates to printing, more particularly to eliminating artifacts in the printing process.
2. Background of the Invention
Electrophotographic (EP) printing appears in many different forms, from desk top printing to photographic development. EP printing typically involves a laser source and a spinning mirror, where the spinning mirror reflects the point laser source onto a photosensitive surface. As the mirror spins, the laser touches the surface at various points, line after line, forming an image on the surface. The surface moves relative to the source.
The surface becomes charged at the point where the light touches it. This charge can then be transferred to a substrate. In some cases, such as in printed circuit board manufacture, the photosensitive surface is merely a layer on the final substrate, in which case the surface and substrate are the same thing. The substrate (paper, film or plate) travels by a developing system which develops the latent image into a visible image. In a printer, the toner forms the image on the paper and is fused to the paper, making the image permanent.
Spatial light modulators can substitute for the spinning mirror. These modulators normally have arrays of individual elements, each representing a pixel in the final image, where either the ON or OFF elements transfer light to the surface. Whether the system transfers light by ON elements or OFF elements is determined before the system is designed. Spatial light modulators have the advantage over the laser and spinning mirror system in that they can perform time delay and integration (TDI) printing easily.
TDI printing involves coordinating the flow of data down the lines of the array with the movement of the photosensitive surface. For example, suppose a device had an array that had however many number of columns necessary to fill a page and sixteen rows. In the first exposure sequence, the first line would display data for line A on the photosensitive surface. In the second exposure sequence, the second line would have the data for line A, since line A on the surface moves as the surface moves. This process repeats until all 16 lines of the device have had an opportunity to expose the surface at line A.
For added control over gray scale, the light source can be modulated with several different power levels, or further phases of exposure can be chosen. A phase is created by using the movement of a photoreceptor relative to the spatial light modulator pixel to create four portions of the pixel's image, allowing spatial modulation within each pixel. Each portion is referred to as a phase. The selection of power levels and the number of lines of exposure for each pixel gives even finer control over gray scale, which will be referred to as time integration for gray scale, or TIG. U.S. patent application Ser. Nos. 08/038,398, "Process and Architecture for DMD Printer," and 08/038,391, "Gray Scale Printing Using Spatial Light Modulators," discuss this control.
The TIG approach involves a look-up table that specifies an exposure bit pattern for each level of gray scale. For simplicity assume that there is no phase modulation in the system, but only light intensity modulation. The discussion is easily generalized to the case where both are present. Assume the repeating intensity pattern is r rows long and there are a set number, n, some multiple of r, rows on the modulator. The look-up table specifies the exposure pattern to expose pixels as if every pixel were on the first row of the modulator. For all lines of data for rows other than the first row on the modulator, the pattern must be rotated left by I mod n, allowing alignment of the exposure pattern with the correct light intensity.
However, this approach results in bands every n lines. The exposures for a pixel are relatively spread out over time in this approach. The spread is different for each pixel position, but the relative spread is the same every n lines. If the data loading on the modulator is not perfectly synchronized to the motion of the drum, bands appear every n lines especially in areas with constant gray scale. Some method, therefore, is needed to allow implementation of TIG, minimizing the artifacts.